In the past, nonwoven webs of meltblown fibers formed using conventional techniques have been considered to be relatively isotropic, especially when compared to nonwoven webs such as, for example, bonded carded webs. The isotropic properties of nonwoven meltblown fiber webs have been considered advantageous in situations where nonwoven web must withstand forces applied in more than one direction.
However, in some situations nonwoven webs of meltblown fibers are subjected to forces applied in only one direction. Thus, it would be desirable to have a nonwoven web of meltblown fibers that is anisotropic. That is, the nonwoven web of meltblown fibers could have different physical properties (e.g., strength, and/or stretch and recovery) in different direction. For example, it would be desirable to have a nonwoven web of meltblown fibers possessing specified levels of physical properties in only the direction that those properties were needed.
An exemplary situation where such an anisotropic nonwoven web of meltblown fibers would be desirable is in certain types of elastomeric composite materials referred to as stretch-bonded laminates. A stretch-bonded laminate is made by joining a nonelastic material to an elastic sheet while the elastic sheet is in a stretched condition so that when the elastic sheet is relaxed, the nonelastic material gathers between the locations where it is bonded to the elastic sheet. The resulting material is stretchable to the extent that the nonelastic material gathered between the bond locations allows the elastic sheet to elongate. An example of this type of material is disclosed, for example, by U.S. Pat. No. 4,720,415 to Vander Wielen et al., issued Jan. 19, 1988.
In many applications, stretch bonded laminates are adapted to stretch and recover in only one direction such as, for example, the machine direction. Thus, the elastic component of the laminate does not have to be isotropic. That is, the elastic component need not have the same stretch and recovery properties in every direction. Desirably, the elastic component would have the required stretch and recovery properties in only the direction that the gathered nonelastic material allows the laminate to stretch. For example, if the fibers of an elastomeric wed of meltblown fibers were generally aligned in only one direction to provide a specified measure of one or more physical properties, such as tension, in that one direction, then relatively fewer elastomeric meltblown fibers could be used than if the web was isotropic. Since elastomeric materials generally tend to be quite expensive, reducing the amount of elastomeric material while still achieving the desired physical properties would be desirable. This is an important consideration since nonwoven webs of meltblown fibers can be used as economical and efficient substitutes for woven or knit textile materials and, in some cases, nonwoven materials such as bonded carded webs. For example, nonwoven webs of meltblown fibers are particularly useful in certain applications in garment materials, pads, diapers and personal care products where an item may be manufactured so inexpensively that it may be economical to discard the product after only one or a limited number of uses.
Although anisotropic nonwoven webs of meltblown fibers are disclosed by U.S. Pat. No. 4,656,081, those webs can be characterized by a heterogenous arrangement of fibers and fiber bundles. In particular, that patent discloses a material having a heterogenous organization in that yarn-like fiber bundles outnumber the fine fibers on one surface of the material and fine fibers outnumber the yarn-like fiber bundles on the other surface of the material. While U.S. Pat. No. 4,656,081 indicates that the material may be made by melt-blowing processes, the heterogenous nature of the material and the presence of yarn-like fiber bundles indicate relative poor web formation which may yield poor web properties that offset any advantage obtained by orienting the fibers.
Accordingly, there is still a need for an anisotropic nonwoven web having a substantially homogenous arrangement of meltblown fibers generally aligned in one of the planar dimensions of the web. Additionally, there is still a need for an inexpensive composite elastic material which is suited for high-speed manufacturing processes and which contains an elastic component that provides the desired elastic properties to the composite only in the one direction of stretch and recovery.